Does exercise really help you lose weight and keep it off? Researchers have mixed opinions –

It is estimated that the global fitness industry will generate more than $80 billion in revenue by 2023. And why not, given the many excellent reasons to exercise? Better cardiovascular health, lower risk of type 2 diabetes, stronger immune system – the list goes on.

One of the biggest reasons many people choose to exercise is to lose weight. As a biobehavioral scientist, I study connections between behavior and health, and I follow the old advice that eating less and exercising more are necessary to lose weight. But a recent debate in the scientific community highlights the growing suspicion that the “exercise more” part of this advice may be incorrect.

At the center of the debate is the limited total energy expenditure hypothesis, which states that exercise will not help you burn more calories overall because your body will compensate by burning fewer calories after your workout. So exercise won’t help you lose weight, even though it will benefit your health in countless other ways.

Obesity researchers object to this hypothesis because it is based on observational research rather than randomized controlled trials, or RCTs, the gold standard of scientific evidence. In RCTs, participants are randomly assigned to either a treatment or a control group, allowing researchers to determine whether the treatment is having an effect. Randomized controlled trials have shown that exercise leads to weight loss.

The verdict is actually more mixed when we look at all the available gold standard evidence.

What the evidence says

Observers of this hypothesis have emphasized the importance of systematically reviewing the evidence from all gold standard studies. They pointed to a 2021 review of more than 100 exercise studies examining the effect on weight loss in adults of high-intensity aerobic, resistance, or interval training, in combination or alone. The review concluded that supervised exercise regimens cause weight loss, even if only a modest amount.

So that settles the debate, yes? If you eat too much dessert, you can just do an extra run to burn off those extra calories, right?

Well, not exactly.

If extra physical activity generally burns extra calories, then exercise should also prevent weight gain after a low-calorie diet. But keeping those lost pounds off after dieting is a common challenge. The same 2021 review contains the few randomized controlled trials that address the question of whether exercise facilitates weight maintenance. However, the results were not as good as for weight loss. The researchers found that six to 12 months of aerobic exercise, resistance training, or both after dieting failed to prevent weight gain in adults.

Practice adherence

But what about compliance? Did all the people in those studies actually exercise regularly?

The 2021 review found only one randomized controlled trial of weight maintenance that reported an objective compliance rate, meaning each exercise session was supervised by a trainer. This tells us the percentage of time that study participants actually exercised as prescribed.

In that trial, the compliance rate was only 64% for 25 postmenopausal women who completed a resistance training program after diet-induced weight loss. This was for a regimen that required participants to exercise two to three times a week for a year. From the perspective of keeping a program for that long, doing so 64% of the time doesn’t seem like a big deal.

But they still gained the same amount of weight as the 29 women in the control group who did not participate in the exercise program.

Energy balance

Many people would say it’s all about balancing energy from food and energy from exercise. If exercise didn’t keep the weight off, maybe a bigger dose of exercise was needed.

The American College of Sports Medicine highlighted this exercise dose issue in its 2009 position statement on physical activity for weight maintenance, stating that the amount of physical activity required for weight maintenance after weight loss is uncertain. In addition, it states that there is a lack of randomized controlled trials in this area that use state-of-the-art techniques to monitor participants’ energy balance.

Fortunately, some of the authors of the position statement used state-of-the-art techniques to monitor energy balance in their own randomized controlled trial. In 2015, they enrolled overweight adults on a 10-month aerobic exercise program and compared the energy intake of those who lost weight with the energy intake of those who did not lose weight during the program. They found that those who don’t lose weight do indeed take in more calories.

Mystery of the disappearing calories

But there’s something else in the energy measurements of that 2015 study that’s quite interesting. At the end of the study, the number of total daily calories the athletes burned did not differ significantly from what the non-athletes burned. And this despite the fact that trainers verified that the exercisers burned an additional 400 to 600 calories per session during their almost daily training sessions. Why didn’t those extra exercise calories show up in the total daily calories burned?

The answer to that question may help explain why exercise doesn’t always help you maintain your weight: Your metabolism responds to regular exercise by reducing the number of calories you burn when you’re not exercising. That is according to the limited total energy consumption hypothesis that fueled the current debate.

Researchers recently tested the hypothesis by measuring the non-exercise calorie burn of 29 obese adults over a nearly 24-hour period, both before and after a six-month exercise program. They found that the calories they burned when they weren’t exercising decreased after months of regular exercise — but only in those who were prescribed the higher of two different exercise doses.

Those who exercised on the lower general health dose, meaning they burned an extra 800 to 1,000 calories per week, saw no change in their metabolism. But those who exercised to lose weight or lose weight on the higher dose, meaning they burned an extra 2,000 to 2,500 calories per week, had a decrease in their metabolism at the end of the study.

Exercise for health

Perhaps both sides of the debate are right. If you want to lose a modest amount of weight, a new exercise routine can make a modest contribution to achieving that goal.

However, as others have said, don’t fool yourself into thinking you can “avoid a bad diet” simply by exercising more. There is a diminishing marginal return on exercise – you end up taking off less weight for the extra exercise you do.

But even if extra exercise may not help you lose and keep it off, there are still the other great health benefits regular exercise provides.

This article first appeared on The Conversation.

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